The invention relates to an avalanche photodiode having separate absorption and multiplication layers.
Among photodetectors, avalanche photodiodes are attractive due to the characteristics high sensitivity and fast response. As could be well-known, the most important factor for producing a highly sensitive avalanche photodiode is the signal to noise ratio. The value of the signal to noise ratio depends upon an impact ionization rate in an avalanche region. Producing a highly sensitive avalanche photodiode requires a high impact ionization rate. Avalanche photodiodes made of compound semiconductors include Ge or InP/InGaAs heterostructure. The signal to noise ratio values of the above conventional avalanche photodiodes are approximately ranged from 1 to 3 only. Much higher impact ionization rate is required to obtain a much higher sensitive avalanche photodiode.
In the prior art, it is known that the impact ionization rate of the avalanche region in the avalanche photodiode can be enhanced providing a superlattice structure therein so that electrons are accelerated traveling from a barrier layer into a quantum well layer. The electrons may receive an energy corresponding to a conduction band gap discontinuity when traveling from a barrier layer into a quantum well layer. The accelerated electrons may contribute to an improvement in the impact ionization rate. One of the conventional superlattice avalanche photodiodes is disclosed in Electronics Letters 5th Jun. 1980 Vol. 16 No. 12 pp. 467-469. Another type of conventional superlattice avalanche photodiodes is disclosed in Applied Physics Letters Vol. 47(6), September 1985, pp. 597-599. A result of the measurement of the impact ionization rate was reported by Kagawa et al. in Applied Physics Letters Vol. 55. 1989, pp. 993-995. In the above publications, it was reported that the superlattice structure such as InGaAs/InAlAs can improve the impact ionization rate. However, such improvement is absent in a bulk structure.
Such conventional avalanche photodiodes having the superlattice structure in the avalanche region are however entangled with serious difficulties as described below. The superlattice structure in the avalanche region involved in the conventional avalanche photodiode has a step-like energy band gap profile or energy band gap discontinuities at interfaces of the well layers and the barrier layers. As described above, when carriers travel from the barrier layer having a large energy band gap into the well layer having a small energy band gap, the carriers may receive an energy corresponding to the energy band gap discontinuity so that the carriers are accelerated for improvement in the impact ionization. By contrast, however, when the carriers travel from the well layer toward the barrier layer biased in the reverse direction, the carriers are forced to experience a potential barrier due to a large energy band gap discontinuity, thereby the carriers tend to accumulate in the well layer. This may impede the avalanche photodiode to obtain a fast response. The normal superlattice structure having the step-like energy band gap profile is unable to permit the avalanche photodiode to obtain a high speed performance or a fast response.
To solve the high speed performance problem of the avalanche photodiode, it is important to reduce or remove an energy band gap discontinuity constituting a potential barrier against carders. It is disclosed in IEEE Transaction on Electron Devices, Vol. ED-30, No. 4, April 1983 to compositionally grade an avalanche region to reduce a potential barrier against electrons when the avalanche photodiode is reverse-biased.
The above compositionally graded avalanche photodiode, however, has a disadvantage because of an existence of a potential barrier against holes. In practice, when photons are absorbed into an absorption region in the avalanche photodiode, electron-hole pairs are generated. The generated electrons and holes travel toward the cathode and anode electrodes respectively. One may assume that when the electrons are travelling through the avalanche region, the holes are traveling in the opposite direction to that of the electrons. The holes experience a step like potential barrier due to a valance band discontinuity. Thus, some holes tend to accumulate in the step like potential barriers due to the valance band discontinuity. The accumulation of the holes at the step like potential barrier may impede the holes to travel toward the anode electrode thereby delaying the arrival of holes at the anode electrode. This translates to a delay in transmission of signals at the opposite electrodes or the cathode and anode electrodes. Therefore, this leads to a difficulty in allowing the avalanche photodiode to obtain a high speed performance. This is a very serious problem.
It is therefore necessary to have the electrons and holes traveling through the avalanche region free from any step-like potential barrier due to conduction or valance band discontinuities, thus, permitting the avalanche photodiode to have a fast response and keeping a high sensitivity level due to a high impact ionization rate.